[ENHANCEMENT] support alternate roms, bios, and saves directories

[udance4ever]

in my Sega to create a unified SHARE directory that works in Batocera Linux and RetroBat (and eventually macOS!), it would be useful to have an option to relocate the roms directory.

So to be clear, instead one monolithic SHARE directory (which requires NTFS to support symlinks), I have my roms directory stored on a separate partition that is exFAT. In this way, the roms directory is now read/writeable in all operating systems (Batocera Linux, RetroBat for Windows and even macOS). This is important since the gamelist.xml files need to be writable to update metadata.

What I've tried

Batocera Linux has support for specifying any partition and relocating the roms directory (and bios and saves for the record), so I have the following in batocera-boot.conf:

sharedevice=DEVICES
sharedevice_part1=ROMS@96A8-CA65:/roms

everything is fine and dandy in Batocera Linux.

if I come back to RetroBat for Windows, I created an NTFS symlink for the roms directory to the exFAT partition.
again, RetroBat is happy.

The issue arises if you boot back into Batocera Linux - Linux finds /userdata/roms is a file (aka symlink), bails and refuses to create /userdata/roms as a mount point and mount on boot. Batocera ES comes up with an error that it can't find any systems.

Possible Solution(s)

ES-DE implements a %ROMPATH% variable (I don't believe Batocera ES has implemented this?)
https://gitlab.com/es-de/emulationstation-de/-/blob/master/USERGUIDE.md#placing-games-into-non-standard-directories

In the spirit of Batocera Linux, it makes sense to support relocating the bios, roms, and saves directories independently. Relocating the roms directory is highest priority to test solving the primary issue first.

Perhaps add option(s) in retrobat.ini (defaulting to current location if option isn't in INI file):
https://wiki.retrobat.org/advanced-features/retrobat.ini